Transformers are often used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits. Transformers are used in a wide variety of devices and processes, such as to provide electric power transmission over long distances, high-voltage direct-current HVDC power transmission systems, and for electric arc furnaces used in steelmaking. Rotating transformers are designed so that one winding turns while the other remains stationary to pass power or radio signals from a stationary mounting to a rotating mechanism, or radar antenna, to pass power or signals from a stationary mounting to a moving part such as a machine tool head. They can also be used for isolating and linking different parts of radio receivers and audio amplifiers, converting high current low voltage circuits to low current high voltage, or vice versa, and balanced-to-unbalanced conversion such as in radio and audio circuits to convert between balanced circuits and unbalanced transmission lines such as antenna downleads. While providing a myriad of electrical benefits and having many uses, transformers belong to a class of expensive components in an electric power system.
One of the problems that plague transformers is that they are subject to inrush currents when they are switched on. Inrush currents are instantaneous currents flowing in the transformer primary circuit when it is energized. These inrush currents are of short duration, unbalanced, high magnitude, high harmonic content and a dc component. An important facet of energizing power systems is the control of the inrush current that is generated and distributed within the system.
Multiple solutions to the problem of inrush current have been used over the years. These include having multiple drive circuits—one for large current and one for small current; incorporating an inrush current limiting resistor and a bypass capacitor in parallel with each other collectively placed in series with the bulk capacitance; adding a pair of transistors and a control circuit for activating at least one of the transistors for a period of time to reduce inrush current; controlling the inrush current by phasing up the input voltage in a controlled manner; and non-simultaneous three phase switching designed to reduce inrush currents. The conventional methods suffer from high resistance values and a great amount of loss associated with that high resistance, as well as uncertainty in residual flux and closing time scatter. Also, these methods become somewhat impractical in a home or office environment and, also, with audio and visual equipment.
Other disadvantages associated with inrush current include faulty operation and failures of electrical systems, electrical and mechanical vibrations among windings in the transformer, irregular voltage distribution along the transformer windings, high amount of voltage drop at the power system at energization times, resonances in the power system resulting from the varied frequencies of the inrush current, current disturbances, increase in harmonics in the system and lower power quality characteristics, and unwanted tripping of protective relays and fuses.
In dealing with these types of current issues, designers typically make tradeoffs between the capacitor or transformer values to minimize noise and/or EMI present in power supplies generated by switching and/or digital circuits and the management of turn-on and inrush currents.
Further, many attempts have been made to control or limit the inrush while not addressing, but only referring to, the underlying cause of the inrush—primarily the effects of the residual flux and closing time scatter.
Thus, a need exists for a circuit that can be used to allow transformers that operate equipment to be able to be started while reducing inrush current to a level compatible with a home or office environment by addressing the effects of the residual flux and closing time scatter.